In recent years, as a printing plate for off-set printing, a large number of CTP systems have been developed and put into practical use, which imagewise exposes directly to a planographic printing plate material employing laser light based on digital image data to record an image on the material. As the CTP systems, there are a silver salt system, a radical polymerization system and a thermal system, which differ in characteristics. With development of a solid laser or semiconductor laser with high output power which emits near infrared light, CTP systems recording with near infrared light, so-called thermal CTP systems has been developed and widely applied.
A planographic printing plate material (printing plate material for CTP) used in the thermal CTP is disclosed in U.S. Pat. No. 5,340,699 which comprises an image recording layer containing an infrared absorbing agent, an acid generating agent, a resol resin, and a novolak resin. In this printing plate material for CTP, imagewise exposure produces an acid is imagewise produced at exposed portions due to interaction of the infrared absorbing agent and the acid generating agent, and heat treatment which follows cross-links the resol resin by action of the acid. When the resulting material is developed with an alkali developer, the unexposed portions having higher alkali solubility are removed with an alkali developer, while the exposed portions (cross-linked portions) lose alkali solubility and imagewise remain to form an image. This system requires heat treatment which heats at 140 to 200° C. for 50 to 120 seconds for accelerating the cross-linking reaction at exposed portions, and has problems in energy consumption and processing time.
Japanese Patent O.P.I. Publication No. 2001-175006 discloses a process comprising the step of (a) imagewise exposing to infrared laser a negative-working image formation material comprising an image recording layer containing an infrared absorbing dye, a radical generating agent, a radically polymerizable compound, and a binder polymer, and (b) then heating the resulting material at 60 to 120° C. for 1 to 20 seconds, wherein heating treatment is carried out after exposure. This process, although it conducts heat treatment at relatively low temperature for relatively short time, improves printing durability. However, the heat treatment is required, and a printing plate material obtained by the process does not provide satisfactory printing durability.
In order to increase efficiency in a plate-making process for preparing a printing plate, a planographic printing plate material with high sensitivity has been sought. Japanese Patent Publication No. 2002-537419 discloses a planographic printing plate material comprising an image recording layer containing a specific polycarboxylic acid. This planographic printing plate material provides high sensitivity without lowering storage stability, but still requires heat treatment at 90° C. for about 60 seconds.
Japanese Patent O.P.I. Publication Nos. 2002-278057 and 2003-5363 disclose a printing plate material comprising a specific cyanine dye as an infrared absorbing agent, in which polymerization efficiency is increased and printing durability is increased without heating. This material provides a certain effect but needs to be improved in printing durability.
A printing plate material comprising a specific onium salt as a radical generating agent is disclosed (see for example, Japanese Patent O.P.I. Publication Nos. 2002-341519 and 2003-76010) in which polymerization efficiency is increased and printing durability is increased without heating. This material also provides a certain effect but needs to be improved in printing durability.